A biaxially oriented polyester film typified by a polyethylene terephthalate film is used for various application purposes, particularly for magnetic recording media, due to its excellent physical and chemical properties.
The slipperiness and abrasion resistance of a biaxially oriented polyester film are important factors that affect the workability in the producing step and processing step of a film and, further, the product quality of the film. If a film lacks these properties, when a biaxially oriented polyester film is used in a magnetic tape by forming a magnetic layer on the surface, friction and abrasion between a coating roll and the film surface are large, and the film is readily wrinkled and scratched on the surface. When the film is used for a VTR or data cartridge, friction between the film and many guide portions, reproduction head and the like occurs at the time of drawing out of the cassette or the like, winding or other operation, whereby the film is scratched or distorted and white powder is formed, for example, by abrasion of the surface of the base film, thereby causing a drop out in many cases.
As a solution to these problems, JP-A 62-172031 proposes a method for adding silicone resin fine particles. This method has a great improving effect and is expected to develop as a future technology.
However, even this method encounters such a new problem as an increase in the amount of white powder generated under severe conditions including high-speed processing for improving productivity such as the formation of a magnetic layer or calendering in the recent production process of a video tape, high-speed dubbing of a soft tape, repetitions of traveling and rewinding and the like.
Further, conventional silicone resin fine particles contain a large number of coarse particles or agglomerate particles. For instance, when they are used in a base film which is required to have high electromagnetic conversion characteristics, there is such a problem that coarse protrusions called fly speck are frequently formed.